Current local area networks allow various equipment to be interconnected, such as computers, data storage devices, audio and/or video playback devices, data receiver devices, printers, routers, landline and mobile telephones, personal assistants, and communications interfaces. This ability to interconnect nearby equipment and to implement sometimes complementary functions produces a growing need for interoperability between equipment connected to the same network.
Interoperability allows equipment to communicate with other equipment and to make functionality available to a user while ensuring simplicity of implementation. For example, it is convenient to be able to connect a printer to a local area network and to be able to print a document from a computer that is connected to the same network without having to first configure a set of parameters used for printing. To do this, there are interoperability standards that, implemented, allow equipment to automatically be detected after being connected to a network and to communicate its characteristics to other equipment, such as its make, model, main function, capacity, or even services that can be implemented through it.
One interoperability standard is UPnP (“universal plug and play”, UPnP is defined by a set of documents available at http://upnp.org/). UPnP is a standard whose purpose is to allow equipment to easily connect to a local area network and to simplify its implementation on the network. UPnP-compatible equipment also supports file sharing, communication, and data interchange for utilities and entertainment applications. UPnP enables the interoperability of equipment with greater simplicity for the user by defining and publishing UPnP control protocols based on the usual communication standards for networks. UPnP AV is the part of the standard dealing with audio and video aspects. The current version of UPnP AV is UPnP AV:4.
With UPnP, it is possible to see from a computer, for example, files stored on a storage space on remote equipment that has been connected to the same local area network as the computer, without having to configure either piece of equipment. Similarly, it is possible to begin playback of audio/video content stored in a file saved on a storage space of a remote device, such as a hard drive, from a playback device, such as a television that is compatible with the content format, as long as both pieces of equipment were connected to the same local area network without any other configuration.
In the case wherein separate files, stored on a hard drive, contain separate components with the same audiovisual content, for example, UPnP can synchronize their respective playback on one or more playback devices. The same content can be a television program, and the components can be the audio and video or the videos corresponding to two different views of the same scene. Synchronization is possible if the components each contain temporal information related to their playback and the pieces of equipment share a common clock. Sharing a common clock applies to equipment connected to the same local area network. The common clock for a local area network is commonly called a “wall clock”.
Methods for saving content allow that content to be played back from one or more files stored on a local server, the playback being specified, for example, according to a UPnP standard. However, these methods have the disadvantage of not allowing access to streamed content, such as audiovisual content transmitted by DTT (Digital Terrestrial Television), by satellite, or even multicasted on a broadband network.